The BMW K100 was the German answer to the surge in popularity surrounding inline-4 motorcycles pouring out of Japan in the ’70s and ’80s. They knew they needed a large displacement 4-cylinder to supplement their ageing boxer twins – and potentially replace them altogether.

They settled on a twin cam inline-4 with fuel injection, a 5-speed transmission, and a shaft drive to the rear wheel. Because the engine is laid on its side, the crank runs along the right side through the gearbox and into the shaft drive, requiring only a single 90° bevel drive to turn the rear wheel. This arrangement reduces powertrain losses, and the low slung engine gives the K100 a low centre of gravity.

BMW sold the K-series bikes in solid numbers, though never quite matching their Japanese counterparts. Over the course of its production run, the K-series bikes would be offered in engine sizes from 750cc all the way up to 1300cc, and today they’re popular choice with custom bike builders due to their bulletproof engineering and unusual looks.

The bike you see here is the work of Matteo Stefanelli, a Belgian engineer with a love of old motorcycles and modern engineering. He acquired a 1985 BMW K100 from an older gentleman who used to delight in riding it at 200 km/h around the famous Spa-Francorchamps circuit near his house – on regular street tires.

When Matteo got his hands on the bike it was filthy, but under the oil and grime it was in surprisingly good shape. After stripping it back, he began the process of designing and fabricating new parts to keep the bike as light as possible, and keep the unusual engine and powertrain front and centre.

He fitted a new BSK Speedworks exhaust with a Supertrapp slip-on muffler, he reinforced the frame and removed unused tabs before sending it off for red powder coating, and he modified the fuel tank.

The original instrument cluster was swapped out for a smaller Motogadget unit, the stock indicators were swapped out for Rizoma LED units on the front and Motogadget blinkers on the back. A custom aluminium seat was made bespoke, along with the front and rear fenders. The completed bike is a solid modern custom that does a great job of showcasing the bones of the original K100, and the good news is that Matteo is just getting started – with more bikes due in 2017 and beyond.

If you’d like to see more from Matteo and Society 142 Motorcycle you can click here to visit the website.

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Published by Ben Branch -